Method and apparatus for sampling a plasma into a vacuum chamber

ABSTRACT

A plasma is generated within an induction coil and the plasma is sampled through an orifice into a vacuum chamber for mass analysis of trace ions in the plasma. Arcing at the orifice is prevented by grounding the induction coil at or near its center, thus eliminating ultraviolet noise and reducing average ion energies and ion energy spread, as well as preventing destruction of the orifice. The elimination of arcing at the orifice allows the use of a sharp edge orifice structure to prevent formation of a cool boundary layer over the orifice and also permits direct sampling of the plasma. The direct sampling and the lack of cooling prevent recombination and reaction of the ions with oxygen and improve the response to elements of high ionization potential, increasing the desired ion signal and greatly reducing the presence of oxides which would otherwise complicate the spectrum.

This invention relates to method and apparatus for sampling aninductively generated plasma through an orifice into a vacuum chamber,and to method and apparatus for mass analysis using such sampling. Theinvention will be described with reference to mass analysis.

Mass analyzers for detecting and analyzing trace substances require thations of the substance to .[.by.]. .Iadd.be .Iaddend.analyzed beintroduced into a vacuum chamber containing the mass analyzer. It isoften desired to perform elemental analysis, i.e. to detect and measurethe relative quantities of individual elements in the trace substance.In theory the trace substance can be reduced to its individual elementsby introducing the trace substance into a high temperature plasma, whichproduces predominantly singly charged ions of the elements. The use of ahigh temperature plasma as an ion source has a number of well recognizedadvantages, including the fact that it produces mostly singly chargedions; interference by other elements to the element to be detected isreduced, isotopic information is obtained, and the ionization efficiencyof the source is very high so that numerous ions are produced foranalysis.

However a major difficulty in the past is associated with the fact thatthe plasma is normally operated at atmospheric pressure; the massanalyzer is located in a vacuum chamber, and therefore a sample of theplasma must be extracted from the plasma and directed through a smallorifice into the vacuum chamber. The plasma is at very high temperature(typically 4,000 degrees K. to 10,000 degrees K.) and is a relativelygood electrical conductor. It is found that when a portion of the hotplasma is directed through a small orifice, an arc-like breakdown occursbetween the plasma and the edge of the orifice, destroying the orificeand producing ultraviolet noise which enters the mass analyzer andinterferes with the detection of ions. The effect has been called the"pinch " effect by other workers and it greatly limits the utility ofthe plasma ion source approach.

Attempts have been made to solve the pinch effect problem by boundarylayer sampling. In this solution the orifice through which the plasma issampled is located in a flat surface of a plate which is kept relativelycool. As the plasma plays against the cool plate, it produces a coolboundary layer immediately next to the plate. Ions are extracted throughthe boundary layer rather than from the plasma directly, and since theboundary layer is cool (and therefore is a relatively good electricalinsulator), arcing effects are reduced or eliminated. However, a majordisadvantage to this approach is that the ions present in the plasmatend to recombine and react in the cool boundary layer and to formoxides. The recombination and reaction reduce the number of ionsavailable for analysis, and the oxide formation greatly complicates theanalysis. Therefore the use of a cooled boundary layer for sampling hasserious commercial disadvantages.

Alan L. Gray, at a conference in January, 1982 entitled "1982 WinterConference on Plasma Spectrochemistry" at Orlando, Fla., U.S.A.disclosed the use of a relatively large orifice (which removed thecooled boundary layer), together with staged vacuum chambers, which issaid to eliminate the prior difficulties. However, the results disclosedappear to be applicable only in limited special circumstances. Theapplicant's tests using a similar sampling arrangement and staged vacuumpumping have not reproduced these results.

The invention provides method and apparatus for sampling a plasmathrough an orifice into a vacuum chamber in which the problem of arcingand generation of ultraviolet noise at the orifice is greatly reduced,and in which the problem of recombination and reaction of ions adjacentthe orifice is also reduced. In one aspect the invention providesapparatus for sampling a plasma into a vacuum chamber comprising:

(a) means for generating a plasma, including an electrical inductioncoil having first and second terminals and at least one turn betweensaid first and second terminals, said turn defining a space within saidcoil for generation of said plasma,

(b) a vacuum chamber including an orifice plate defining a wall of saidvacuum chamber,

(c) said orifice plate having an orifice therein located adjacent saidspace for sampling a portion of said plasma through said orifice intosaid vacuum chamber,

(d) .Iadd.said coil constituting the sole electrical power means forgenerating said plasma, .Iaddend.

.Iadd.(e) .Iaddend.and circuit means connected to said coil between saidterminals to reduce the peak to peak voltage swing in said plasma.Iadd.,thus to reduce the likelihood of electrical arcing from saidplasma.Iaddend..

In this description and in the appended claims, the term "vacuumchamber" is intended to mean a chamber in which the pressure issubstantially less than atmospheric.

In another aspect the invention provides a method of sampling a plasmainto a vacuum chamber comprising:

(a) applying a high frequency electrical current to a coil to generate aplasma within said coil,

.Iadd.(b) said current in said coil being the sole electrical powermeans for generating said plasma, .Iaddend.

.[.(b).]. .Iadd.535 (c) .Iaddend.reducing the peak to peak voltagevariations in said plasma by limiting the voltage .[.variation.]..Iadd.variations .Iaddend.in said coil at a position between the endsthereof, .Iadd.thus to reduce the likelihood of electrical arcing fromsaid plasma, .Iaddend.and .[.(c).]. .Iadd.(d) .Iaddend.directing aportion of said plasma through an orifice into said vacuum chamber.

Further objects and advantages of the invention will appear from thefollowing description, taken together with the accompanying drawings inwhich:

FIG. 1 is a diagrammatic view (not to scale) showing prior art apparatusfor mass analysis and with which the invention may be used;

FIG. 2 is a schematic drawing of an impedance matching circuit andinduction coil used with the apparatus of FIG. 1;

FIG. 3 is a schematic drawing similar to that of FIG. 2 but showing animpedance matching circuit and induction coil modified according to theinvention;

FIG. 4 is a graph showing the absolute value of the plasma RF voltageplotted against the position of the tap taken axially along the coil;

FIG. 5 is a graph showing the energy and energy spread of ionstransmitted into the mass spectrometer from the plasma for two positionsof the ground tap of FIG. 3;

FIG. 6 is a further graph showing the energy and energy spread of ionstransmitted from the plasma into the mass analyzer for two positions ofthe ground tap of FIG. 3;

FIG. 7 is a mass spectrum for strontium taken with the ground tap ofFIG. 3 at a first position;

FIG. 8 is a mass spectrum for strontium taken with the ground tap ofFIG. 3 at a second position;

FIG. 9 is a cross sectional view of an orifice plate having a bluntorifice structure;

FIG. 10 is a cross sectional view of an orifice plate having a sharpedge orifice structure;

FIG. 11 is a mass spectrum for cerium taken using the blunt orificestructure of FIG. 9;

FIG. 12 is a mass spectrum for cerium taken using the sharp edge orificestructure of FIG. 10;

FIG. 13 is a graph showing relative numbers of ions versus theirionization potential; and

FIG. 14 shows an alternative electrical circuit for use with theinvention.

Reference is first made to FIG. 1, which shows a plasma tube 10 aroundwhich is wrapped an electrical induction coil 12. A carrier gas (e.g.argon) used to form the plasma is supplied from a source 13 and isdirected via conduit 14 into the plasma tube 10. A further stream of thecarrier gas is directed from the source 13 through an inner tube 15within the plasma tube 10 and exists via a flared end 16 just upstreamof the coil 12. A sample gas containing the trace substance to beanalyzed is supplied in argon from source 17 and is fed into the plasmatube 10 through a thin tube 18 within and coaxial with the tube 15. Thusthe sample gas is released into the centre of the plasma to be formed.

The coil 12 normally has only a small number of turns (four turns in theembodiment tested) and is supplied with electrical power from an RFpower source 20 fed through an impedance matching network 22. The powerfed to the coil 12 varies depending on the nature of the plasma requiredand may range between 200 and 10,000 watts. The energy supplied is athigh frequency, typically 27 MHz. The voltage across the coil 12 isbelieved to be up to several thousand volts, depending on operatingconditions. The plasma generated by this arrangement is indicated at 24and is at atmospheric pressure

The plasma tube 10 is located adjacent a first orifice plate 26 whichdefines one end wall of a vacuum chamber 28. Plate 26 is water cooled,by means not shown. Gases from the plasma 24 are sampled through anorifice 30 in the plate 26 into a first vacuum chamber section 32 whichis evacuated through duct 34 by a pump 36. The remaining gases from theplasma exit through the space 38 between the plasma tube 10 and theplate 26.

The first vacuum chamber section 32 is separated from a second vacuumchamber section 40 by a second orifice plate 42 containing a secondorifice 44. The second vacuum chamber section 40 is evacuated by avacuum pump 46. Located in the second vacuum chamber section 40 is amass analyzer indicated at 48. The mass analyzer may be a quadrupolemass spectrometer having rods 50. For purposes of clarity the plasmatube 10 in FIG. 1 has been shown greatly enlarged with respect to thevacuum chamber.

In use, the first vacuum chamber section 32 is typically maintained at apressure of about 1 torr, and a second vacuum chamber section 40 istypically maintained at a pressure of 10⁻⁵ torr. A portion of the plasma24 is sampled through the first orifice 30 into the first vacuum chambersection 32. Ions in the plasma are drawn through the first orifice 30into the first vacuum chamber section 32 by the gas flow through thefirst orifice 30. The ions are then drawn through the second orifice 44again by the gas flow through the second orifice 44.

As discussed, it is found that when the system shown in FIG. 1 is used,the plasma 24 tends to arc through or to the first orifice 30 andsometimes may even arc through or from the first orifice 30 to thesecond orifice 44. The arcing destroys the orifices and also generatesultraviolet noise which interferes with the analysis of any ions whichmay enter the mass analyzer 48. In addition, ions characteristic of theorifice material may appear in the mass spectrum and interfere with theanalysis.

The undesired arcing is aggravated when (as in the present case) thereis a vacuum chamber 28 on the side of the first orifice plate 26 remotefrom the plasma 24. The increased arcing occurs because the increasedflow of gas through the orifice 30 caused by the vacuum tends to removethe cooled layer of gas which would otherwise tend to collect againstthe outside of the orifice plate 26 and which would provide someelectrical insulation against arcing. If the first orifice 30 is madesufficiently small, then the cooled layer 51 of gas overlying the firstorifice plate 26 at the first orifice will tend to exist even withvacuum pumping, but with a very small orifice 30, only a small sample ofthe plasma 24 can be drawn into the first vacuum chamber section 32,reducing the ion signal. In addition if the first orifice 30 is madevery small it more readily tends to melt or clog. If the first orifice30 is made larger, then the cooled layer 51 of gas overlying the orificeplate 26 becomes thin or vanishes and arcing occurs as indicated.

The applicant has discovered after extensive research that the arcingappears to be caused by large peak to peak voltage swings in the plasmaitself. Although it is difficult to measure voltages in the plasmagenerated by a high frequency electrical field (because the probe usedfor measurement tends to be melted by the plasma and because ofundesirable RF pick-up produced by the generating field), adetermination has been made that the peak to peak voltage swing in theplasma with the arrangement shown is very large (e.g. of the order of upto 1,000 volts). Having made this determination, the next problem was todetermine how this voltage swing was being produced.

Tests were then conducted to determine the origin of the large voltageswings in the plasma, and these tests will be explained with referenceto FIG. 2. FIG. 2 shows a circuit for the typical tuning and impedancematching device 22 used to supply RF power to the plasma. The impedancematching device 22 consists of two variable capacitors C1, C2 connectedin series at terminal 52 with the power source 20 connected acrosscapacitor C1 at terminals 52, 54. A terminal 56 at the free end ofcapacitor C2 is connected to terminal 58 at the upstream end of the coil12 while the other end 60 of coil 12 is connected to terminal 54. Thedirection of gas flow through the coil 12 is indicated by arrow 62. Thearrangement as shown in FIG. 2 produced the very large voltage swingsdiscovered in the plasma 24.

The first test was to connect a ground to terminal 60 immediately at thedownstream end of the coil, on the theory that the long lead used from60 to 54 had inductance which was generating a voltage swing at terminal60 and that this was contributing to the voltage swing in the plasma.This additional ground reduced the voltage swing to less than half ofthat originally detected, but a large voltage swing in the plasmaremained and still produced arcing.

Next, the impedance matching circuit was modified as shown in FIG. 3, sothat the former connection between ground and terminal 54 was removed.Instead the coil 12 was tapped at 64 and the tap 64 was grounded. Thetap 64 was then moved back and forth along the coil and the peak to peakvoltage swings in the plasma 24 were measured for different positions ofthe tap 64 along the coil 12. The measurements are plotted to form curve66 in FIG. 4, where the absolute value of the plasma peak to peakvoltage swing is shown on the vertical axis and the position of the tap64 is shown on the horizontal axis. On the horizontal axis the number"0" indicates the terminal 60 at the downstream or exit end of the coil12, and the number "4" denotes the terminal 58 at the entrance orupstream end of the coil 12. The numbers "1", "2" and "3" indicate turns1, 2 and 3 respectively of the coil 12. The center of the coil islocated at "2" in FIG. 4.

In the FIG. 4 curve it will be seen that at point 68, the tap 64 islocated downstream of terminal 60, between terminals 54 and 60. It willbe seen in FIG. 4 that the absolute value of the peak to peak voltageswing 66 in the plasma decreases as the tap 64 is moved from thedownstream end "0" of the coil toward the center "2" of the coil,reaching a minimum at the two turn location. The voltage swing thenincreases as the tap 64 is moved toward the upstream end "4" of thecoil. The voltage at the null point 70 is indicated as being about 13volts, but it is difficult to measure the voltage accurately to withinless than five volts absolute value because of RF pick-up difficulties.In addition, a small voltage (of the order of 10 volts) is generated inthe plasma by heating currents flowing through the plasma and thisvoltage is apparently not eliminated by moving the tap 64. It will benoted that the voltage measurements shown were of the absolute value ofthe voltage swing in the plasma, because it is difficult to measure thepolarity of such voltage. However, in theory it is expected that thevoltage swings being measured would reverse in phase as the tap 64 ismoved past the center "2" of coil 12.

When the tap 64 was located near the center of the coil (e.g. withinabout one-quarter turn from the center of the coil for a four turncoil), it was found that arcing at the orifices 30, 44 was eliminatedand in addition both the energy and the energy spread of the ionstravelling through the orifices were much reduced. Specifically,reference is next made to FIG. 5, which shows on the vertical axis thenumber of ions travelling through the orifices 30, 44 into the massanalyzer 48, and on the horizontal axis the energies of such ions inelectron volts. Curve 72 shown in solid lines and with solid measurementpoints was produced when the tap 64 was located one-quarter turn fromthe end "0" of the coil, and curve 74 shown in dotted lines and withoutline measurement points resulted when the tap 64 was located at oneand three-quarter turns from the end "0" of the coil (i.e. nearly at thecenter of the coil). For curve 72 considerable arcing occurred throughthe orifice and there was considerable scatter of the observed points,as shown, so a smoothed line was drawn through the points. It will beseen that the energy spread of the ions at 10% height was about 44electron volts and at 50% height was about 17 electron volts. Inaddition the maximum energy of a substantial number of the ions exceeded30 electron volts. The high energies and energy spread of the ionsgreatly reduce the ability of the quadrupole mass analyzer 48 to analyzethe trace substance being examined. In contrast, it will be seen fromcurve 74 that the energy spread of the ions passing through the orificeswas much less, namely about 11 electron volts at 10% height and about 5electron volts at half height. The improvement was dramatic and leads toa corresponding improvement in detection and analysis, as will beexplained.

FIG. 6 is similar to FIG. 5 but shows curve 76 produced when the tap 64was located at three-quarters of a turn from the end "0" of the coil andcurve 78 produced when the tap 64 was again located one andthree-quarter turns from the end "0" of the coil. The results aresimilar to those described previously, i.e. for the tap 64 near thecenter of the coil, both the energy spread of the ions and the averageenergy of the ions are much reduced.

The effect of the reduced ion energy and ion energy spread will beexplained with reference to FIGS. 7 and 8, which are mass spectra for aten parts per million solution of the element strontium. The number ofion counts detected is shown on the vertical axis and the mass in atomicmass units (amu) is shown on the horizontal axis. FIG. 7 shows the massspectrum obtained with the tap 64 located three-quarters of a turn fromthe downstream end "0" of the coil (as shown for curve 76 in FIG. 6).FIG. 8 shows the mass spectrum obtained when the tap 64 is located oneand three-quarter turns from the downstream end "0" of the coil (asshown for curve 78 in FIG. 6). In both cases the full scale value on thevertical axis was 3×10⁴ counts per second. It will be seen that in FIG.8 the three strontium peaks indicated at 80a, 82a and 84a (correspondingto 86, 87 and 88 atomic mass units) have been clearly resolved whereasin FIG. 7 the same peaks 80b, 82b, 84b have been poorly resolved and themaximum level of peak 84b is lower than that of peak 84a. As expected,the reduced ion energies and energy spread have produced substantiallygreater resolution and increased ion signal for analysis.

A further advantage of the invention is that because there is no need tosample from a cool boundary layer used to protect the orifice, theorifice sampling plate may be arranged to reduce or eliminate any suchcool boundary layer. This aspect of the invention is explained withreference to FIGS. 9 and 10. FIG. 9 shows a first orifice plate 26ahaving a blunt conical orifice structure 88 defined by a conical sidewall 89, a flat (i.e. blunt) top wall 90, and an orifice 30a in the topwall 90. In use the blunt top wall 90 tends to produce a cool boundarylayer (as shown at 51 in FIG. 1) of gas over the orifice 30a, whichboundary layer insulates the orifice from the plasma in order to reducearcing. Unfortunately since the plasma is at atmospheric pressure, rapidrecombination and reaction of the ions with oxygen occurs at the coolboundary layer (the recombination rate varies with the third power ofthe pressure and the reaction rate varies with the second power of thepressure). This results not only in loss of ion signal available foranalysis but also in the entrance of oxides into the mass analyzer,complicating the analysis.

FIG. 10 shows an alternative first orifice plate 26b having a sharp edgeorifice structure 92 defined by a conical side wall 94 terminating at a.[.sharpe.]. .Iadd.sharp .Iaddend.edge 96. The edge 96 defines the firstorifice 30b. The FIG. 10 orifice structure results in the reduction orelimination of a cool boundary layer over orifice 30b (even though theplate 26b itself may be cooled), because there is not flat surfaceadjacent the orifice over which a cooled boundary layer can readilyform. Thus the plasma being sampled through orifice 30b is not greatlycooled until after it enters vacuum chamber section 32. Since thepressure in vacuum chamber section 32 is only about one torr (ascompared with 760 torr on the outside of orifice plate 26b), therecombination rate is reduced by about 760³ and the reaction rate byabout 760².

The improvement produced by the use of the sharp edge orifice structure92 (which can be used without arcing because of the tap 64 located nearthe center of the coil) is shown in FIGS. 11 and 12, which show massspectra obtained for a ten parts per million solution of cerium. FIG. 11shows the mass spectrum 98 obtained using the blunt orifice structure 88of FIG. 9 and FIG. 12 shows the mass spectrum 100 obtained using thesharp edge orifice structure 92 of FIG. 10. Here full scale on thevertical axis was 10⁶ counts per second. It will be seen that in FIG. 11the peak at 140 amu (which is the mass of cerium) is extremely small,while a large peak is located at mass 156 (cerium oxide) and a smallerpeak (but still larger than the cerium peak) is located at mass 158 (theoxide of an isotope of cerium).

In contrast FIG. 12 shows a large peak at mass 140 (cerium) and asubstantial peak at mass 142 (an isotope of cerium). Only a small peaknow appears at mass 156 (cerium oxide), and virtually no peak appears atmass 158. The enormous increase in ion signal for the elemental ions andthe corresponding reduction in the quantity of oxides produced greatlyimprove the ability to decipher the complex spectrum obtained when manyelements are mixed together. (For FIG. 12 the resolution wasdeliberately reduced to ensure that there would be no mass discrimationagainst the higher mass oxides.)

A further advantage of the invention is that it improves the response toelements of high ionization potential. Formerly it was common practiceto place an extra water cooled orifice plate between the first orifice30 and the plasma 24. Thus a reduced scale, rapidly cooling plasma wassampled through the first orifice 30. Air mixed rapidly into this plasmaand reacted thereon to produce nitric oxide (NO). The ionizationpotential of NO is 9.25 electron volts. Metal ions of higher ionizationpotential in the plasma tended to undergo change transfer reactions withthe NO to produce NO⁺ and neutral metal atoms. The metal atoms, havingbecome neutral, could not be detected by the mass analyzer.

When the invention is used, sampling may be carried out much closer tothe hot plasma (since arcing has been essentially eliminated) and airhas less opportunity to mix into the plasma sample. Therefore nitrogenoxides are less likely to form. Thus ions of higher ionization potentialdo not lose their charge and hence can be seen by the mass analyzer.This is illustrated in FIG. 13, which shows relative numbers of ions onthe vertical axis on a log scale, and the ionization potential of theelements in electron volts (various elements are marked on the graph) onthe horizontal axis. The curve for the prior art method without the useof the invention is shown at 110 and the curve with the invention usedis shown at 120. For higher ionization potential elements such as zinc,the improvement in ion signal can be by a factor of fifty. For mercurythe improvement is even greater.

It will be realized that although the tap 64 is shown as grounded, itmay instead be clamped to a different fixed potential, depending on thecircuit arrangements provided. Alternatively a variable voltage may beapplied to tap 64, so long as the effect is to reduce sufficiently thepeak to peak voltage swing in the plasma.

As a further alternative the tap 64 may be eliminated entirely and acircuit such as that shown in FIG. 14 may be used. In the FIG. 14circuit the power supply 20 is connected to terminals 54, 56, i.e.across the two capacitors now indicated as C1', C2', and the terminal 52between capacitors C1', C2' is grounded. Terminals 56, 58 are connectedtogether as are terminals 54, 60, as before. Provided that the circuitis carefully balanced so that the capacitance of C1' and its leads isequal to the capacitance of C2' and its leads, the circuit will besymmetrical and will be equivalent electrically to having a groundcentre tap in coil 12. Thus the RF voltage at the centre of coil 12 willremain at or near zero as before.

Impedance matching, if needed for the FIG. 14 circuit, may be effectedby a transformer or other means located between the RF power source 20and the location in the circuit now shown for the source 20.

Although a four turn coil has been shown, more or fewer turns may beused as appropriate for the application in question.

I claim:
 1. Apparatus for sampling a plasma into a vacuum chambercomprising:(a) means for generating a plasma, including an electricalinduction coil having first and second terminals and at least one turnbetween said first and second terminals, said turn defining a spacewithin said coil for generation of said plasma, .Iadd.and means forsupplying an alternating electrical current to said coil to excite saidplasma, .Iaddend. (b) a vacuum chamber including an orifice platedefining a wall of said vacuum chamber, (c) said orifice plate having anorifice therein located adjacent said space for sampling a portion ofsaid plasma through said orifice into said vacuum chamber, .Iadd.(d) theelectrical current in said coil constituting the sole electrical powermeans for generating said plasma, .Iaddend. .Iadd..[.(d).]..Iaddend..Iadd.(e) .Iaddend.and circuit means connected to said coil between saidterminals to reduce the peak to peak voltage swing in saidplasma.Iadd.thus to reduce the likelihood of electrical arcing from saidplasma.Iaddend..
 2. Apparatus according to claim 1 wherein said coilincludes a plurality of turns.
 3. Apparatus according to claim 2 whereinsaid circuit means includes means for holding the potential at a pointin said coil between said terminals at a substantially constant value.4. Apparatus according to claim 3 wherein said value is ground. 5.Apparatus according to claim 3 wherein said point is located at or nearthe centre of said coil.
 6. Apparatus according to claim 4 wherein saidpoint is located at or near the centre of said coil.
 7. Apparatusaccording to claim 2 wherein said circuit means includes a tap connectedto said coil between said terminals thereof, said tap being located ator near the centre of said coil.
 8. Apparatus according to claim 7wherein said tap is clamped to a substantially fixed potential. 9.Apparatus according to claim 8 wherein said potential is ground. 10.Apparatus according to claim 3 and including mass analyzer means locatedwithin said vacuum chamber for analyzing ions sampled into said vacuumchamber from said plasma.
 11. Apparatus according to claim 9 andincluding mass analyzer means located within said vacuum chamber foranalyzing ions sampled into said vacuum chamber from said plasma. 12.Apparatus according to claim 3 wherein said orifice plate includes aconical wall extending outwardly from said plate towards said space,said conical wall having a sharp outer edge which defines said orifice.13. Apparatus according to claim 9 wherein said orifice plate includes aconical wall extending outwardly from said plate towards said space,said conical wall having a sharp outer edge which defines said orifice.14. Apparatus according to claim 3 wherein said orifice plate includes aconical wall extending outwardly from said plate towards said space,said conical wall having a sharp outer edge which defines said orifice,said apparatus further including mass analyzer means located within saidvacuum chamber for analyzing ions sampled into said vacuum chamber fromsaid plasma.
 15. Apparatus according to claim 9 wherein said orificeplate includes a conical wall extending outwardly from said platetowards said space, said conical wall having a sharp outer edge whichdefines said orifice, said apparatus further including mass analyzermeans located within said vacuum chamber for analyzing ions sampled intosaid vacuum chamber from said plasma.
 16. A method of sampling a plasmainto a vacuum chamber comprising:(a) applying a high frequencyelectrical current to a coil to generate a plasma within said coil,.Iadd.(b) said current in said coil being the sole electrical powermeans for generating said plasma, .Iaddend. .[.(b).]. .Iadd.(c).Iaddend.reducing the peak to peak voltage variations .[.is.]. .Iadd.in.Iaddend.said plasma by limiting the voltage variations in said coil ata position between the ends thereof, .Iadd.thus to reduce the likelihoodof electrical arcing from said plasma, .Iaddend.and .[.(c).]. .Iadd.(d).Iaddend.directing a portion of said plasma through an orifice into saidvacuum chamber.
 17. A method according to claim 16 wherein said step (b)comprises holding the potential in said coil at a position between theends thereof at a substantially constant value.
 18. A method accordingto claim 17 wherein said value is substantially ground.
 19. A methodaccording to claim 17 wherein said position is at or near the center ofsaid coil.
 20. A method according to claim 17 wherein said position iswithin one-quarter turn from the center of said coil.
 21. A methodaccording to claim 17 wherein said position is at or near the center ofsaid coil and including the step of analyzing ions in said portion ofsaid plasma.
 22. A method according to claim 17 wherein said position isat or near the center of said coil and including the step of analyzingwith a mass analyzer ions in said portion of said plasma.
 23. A methodaccording to claim 17 wherein said position is at or near the center ofsaid coil and including the step of analyzing ions in said portion ofsaid plasma, said method further including the step of preventingsubstantial cooling of said plasma over said orifice to reducerecombination and reaction of said ions in said plasma with oxygen..Iadd.24. Apparatus according to claim 1 and including means formaintaining said plasma at substantially atmospheric pressure..Iaddend..Iadd.25. A method according to claim 16 wherein said plasma is operatedat substantially atmospheric pressure..Iaddend.